Vitali undoubtedly has the physical dimensions to compete in any era. Is he the greatest of all time; in a word No.
I think we can agree that Lewis was the best fighter he ever fought. Lewis beat him; stopped on cuts is still stopped; end of. You cant call it lucky if a cut is caused by a punch. Some fighters are gifted with an iron chin (Vitali case in point), some are glass jawed. Some fighters get cut easily, some dont. Its a natural characteristic and something you just have to deal with! Weak skinned, weak chinnned, either can end your night fast! After Vitali got cut he had 2 options, go out of his way to protect the eye and try outpoint Lewis or get Lewis out of there asap. With the success he was having, he probably made the obvious choice, but as it turned out it was the wrong choice.
If you want to talk about Ali, for me Ali beats him. A lot of time has been spent talking about how does Ali get to Vitali, but how does Vitali hit Ali? Slower hands and very ponderous foot speed by comparison means he would struggle to land. Also despite having an unbelievable beard, Vitali gets hit. Ali would be able to avoid Vitali's onslaught and outpoint him in a pure demonstration of the sweet science.